The invention relates to the structure of an immersed surface. More particularly, it relates to a means for increasing the critical heat flux of an immersed surface. The invention is not directed toward increasing the heat transfer rate or increasing the surface heat transfer coefficient except that such increases may be incidental to and interdependent with an increase in the critical heat flux.
In many immersion cooling applications, i.e. those in which the component to be cooled is immersed in a cooling medium, the thermal rating of the component being cooled is limited by the critical heat flux. For example, the maximum power rating of power semiconductor modules is limited by the critical heat flux in immersion cooling. The critical heat flux may be defined as the maximum total heat per unit area capable of being transferred from a given surface without an excessive temperature rise.
It has been shown that when vapor jets in the cooling liquid at high heat flux are spaced closer together than their natural spacing or wavelength by the use of baffles above the heating surfaces then an increase in the critical heat flux is obtained. However, in a counter-current liquid vapor flow the maximum vapor velocity in the vapor jets is limited by flooding, which is the point at which the liquid phase can no longer flow to the surface of the object to be cooled because of the quantity of vapor emanating therefrom. It is desired to increase the critical heat flux capability of pool or immersed boiling surfaces while not losing the advantage of higher thermal conductance in a more gradual transition to film boiling thereby delaying the occurrence of flooding.
While the use of finned surfaces can lower the temperature rise with nucleate boiling, such finned surfaces have limited effect on the critical heat flux unless they are appropriately designed. For instance, if the fin is too long, the temperature drop along the fin may be too large. This excessive temperature drop may create a situation where there is nucleate boiling at or near the tip of the fin but where there is a film boiling condition at the base of the fin. The slot surface area between fins also must be limited since if there is too much surface area, too much vapor will be generated in the slots and local flooding will then be reached at a lower heat flux.